Children in the US fall far below current guidelines for daily physical activity (PA), and spend considerable free time in sedentary pursuits (CDC, 2009;Gordon-Larsen, Nelson, &Popkin, 2004). This assessment appears true for our underserved population of children and adolescents with intellectual disabilities (ID), including Down syndrome (DS) (Whitt-Glover, O'Neill, &Stettler, 2006). The health benefits of regular and sustained PA are compelling. If progressive resistance training is combined with aerobic activity, benefits may include improvements in muscular strength, muscular endurance and cardiovascular fitness;control of blood pressure;bone health;reduced risk for overweight/obesity;and psychological wellbeing. PA is listed as a "leading health indicator" in Healthy People 2010. Research with children with ID has demonstrated increases in health-related fitness, but under tightly controlled training conditions. Translational research is needed to begin to determine whether similar improvements in fitness can be achieved under more naturalistic conditions (e.g., in community YMCAs), and, importantly, whether PA and its outcomes are maintained. This proposed study seeks to determine whether adolescents with DS who participate in a 16-week, YMCA- based exercise intervention (INT), as compared with participants in a wait-list control group (WL) will: 1) show greater improvements in indices of muscular strength, muscular endurance and aerobic fitness at 8 and 16 weeks, relative to baseline (primary aim);2) maintain exercise behavior and fitness outcomes over a 16-week post-intervention period, with follow-up measurement at 24 and 32 weeks (primary aim);3) show more favorable changes in body composition (fat-free mass) and BMI, measured at baseline, 16 and 32 weeks (secondary aim);and 4) engage in more generalized PA in and outside of the YMCA, as measured by accelerometry, at baseline, 16 and 32 weeks. Forty (40) participants with Down syndrome, ages 16-22 years, will be randomly assigned to the INT (N=20) and WL (N=20) groups. Measures will include 1-RM bench and leg press for muscular strength, maximum repetitions of 50% 1-RM bench and leg press for muscular endurance, 20m shuttle run for aerobic fitness, bioelectrical impedance for body composition, and accelerometry showing average time in different levels of PA. The INT will be provide supervised training and coaching in weights and aerobic exercise, the formal peer dyads, and focused educational messages on maintaining the exercise program and extending PA to outside of the YMCA. Our long-term programmatic goal is to more effectively address the health disparities noted in the ID population;the present R21 project was designed provide the foundation for accomplishing our long-term goal. PUBLIC HEALTH RELEVANCE: The US Department of Health and Human Services has identified major heath disparities among children, adolescents, and adults with intellectual disabilities, including those with Down syndrome. There is a need for more community-based health promotion programs for individuals with DS in order to offset the elevated risk factors for chronic disease observed in this underserved population. Physical activity is listed prominently as a leading health indicator in Healthy People 2010, and objectives include improving activity levels and reducing sedentary behavior in children with disabilities.